The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known powertrain architectures include torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to an output member. One exemplary powertrain includes a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving motive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque thereto. Electric machines, operative as motors or generators, generate a torque input to the transmission, independently of a torque input from the internal combustion engine. The electric machines may transform vehicle kinetic energy, transmitted through the vehicle driveline, to electrical energy that is storable in an electrical energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating state and gear shifting, controlling the torque-generative devices, and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed.
Operation of the above devices within a hybrid drive vehicle require management of numerous torque bearing shafts or devices representing connections to the above mentioned engine, electrical machines, and driveline through an output shaft. Various control schemes and operational connections between the various aforementioned components of the hybrid drive system are known, and the control system must be able to engage and disengage the various components in order to perform the functions of the hybrid drive system. Engagement and disengagement are known to be accomplished through the use of a transmission employing clutches. Clutches are devices well known in the art for engaging and disengaging shafts including the management of rotational velocity and torque differences between the shafts. Engagement or locking, disengagement or unlocking, and operation while engaged or locked operation are all transmission states that must be managed in order for the vehicle to operate properly and smoothly. These implications to vehicle operation are collectively described as part of a vehicle characteristic called drivability.
Clutches are known in a variety of designs and control methods. One known type of clutch is a mechanical clutch operating by separating or joining two connective surfaces, for instance, clutch plates, operating, when joined, to apply frictional torque to each other. One control method for operating such a mechanical clutch includes applying a hydraulic control system implementing fluidic pressures transmitted through hydraulic lines to exert or release clamping force between the two connective surfaces. Operated thusly, the clutch is not operated in a binary manner, but rather is capable of a range of engagement states, from fully disengaged and desynchronized, to synchronized with no clamping force applied, to engaged but with only minimal clamping force, to engaged with some maximum clamping force. Clamping force applied to the clutch determines how much reactive torque the clutch can carry before the clutch slips. Clutches can be designed to operate with some level of controlled slip in asynchronous operation, or clutches can be designed to operate with little or preferably no slip in synchronous operation. This disclosure deals with clutches designed primarily for synchronous operation. Variable control of clutches through modulation of clamping force allows for transition between locked and unlocked states and further allows for managing slip in a locked transmission. In addition, the maximum clamping force capable of being applied by the hydraulic lines can also vary with vehicle operating states and can be modulated based upon control strategies.
Slip, or relative rotational movement between the connective surfaces of the clutch, occurs whenever the reactive torque transmitted through the clutch exceeds the actual torque capacity created by the applied clamping force. Clutches can be asynchronous, designed to accommodate slip, or clutches can be synchronous, designed to operate with little or no slip. This disclosure is related primarily to synchronous clutches. Slip in a transmission in synchronous operation results in unintended loss of control within the transmission and adverse affects to drivability.
Transitioning from a locked to an unlocked state requires a transitional unlocking state, through which a clutch begins in a locked, synchronized state with connective surfaces clamped together and ends in an unlocked, substantially separated state with no reactive torque being transferred or carried across the clutch. Clamping force applied by a clutch control system transitions in the unlocking state from a clamping force sufficient to transfer a demanded torque across the clutch without slip to a fully released clamping force, sufficiently lowered to create an unlocked state. Orderly release of the clamping force through the unlocking state, facilitating optimal drivability of the vehicle, can be difficult to achieve. As described above, whenever the reactive torque transmitted through the clutch exceeds the actual clutch capacity created by the applied clamping force, slip occurs. Because unlocking necessarily involves a rapid decrease in clamping force, engine and electric machine torques creating too much reactive torque across a clutch during an unlocking event create risk to drivability as potential slip.